Obviously the real life reason is that Enterprise was made after those other series.

Other than that though I wondered if anything had ever been put in canon (or even quasi-canon, like a producer or writer or one of the ST bigwigs making a statement in an interview or what have you) by way of explaining why they are never seen "again".

Lots of species in (and out) of the Federation. The fish people wouldn't serve on a Humanoid starship. The reptile people might be physiologically incapable of interacting with other species aboard a ship.

How many Bolians did we see on Kirk's ship? How many Andorians on Janeway's?

I guess for the same reason that the Federation had forgotten about temporal travel. In TOS, there was very little time travel as it was new and involved a sling shot around the sun. But in Enterprise it covered time travel a lot including a temporal cold war.

So, perhaps all this time leapingcaused the Federation to forget the Xindi or they got lost in time?

I've wondered why the Denobulans weren't among the founding members of the Federation. Denobula is apparantly close to Earth, they were shown working closely with humans in several episodes, and they even had an ambassador present during the negotiations in "Terra Prime". Yet they are never seen again.

I've wondered why the Denobulans weren't among the founding members of the Federation. Denobula is apparantly close to Earth, they were shown working closely with humans in several episodes, and they even had an ambassador present during the negotiations in "Terra Prime". Yet they are never seen again.

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Maybe there were some issues in the Federation Charter that the Denobulan government didn't agree with, so they held off on joining up right away.

Maybe there were some issues in the Federation Charter that the Denobulan government didn't agree with, so they held off on joining up right away.

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Given what we've seen in future shows, like TNG and DS9 for example, the Federation becomes entirely based on human values...

take genetic modification for example... which is outlawed by the Federation, even though Denobulans used it for centuries before they even met humans...

Or it could be that the strictly human basis for ideals and such, was at conflict with the obviously polyamorous nature of the Denobulan society... even Trip had a tough time getting his head around that, and if the Federation is based as much on human society, ideals, and laws as it seems to be in future shows, then i doubt the Denobulans would be very comfortable there...

The Xindi have a habit of trying to commit genocide against random species that they've never met for ridiculous and nonsensical reasons. Wonder how the Klingons or Romulans would react to their shenanigans.

Personally, I think Daniels was full of it, the Xindi are extinct, and the trinket Daniels stole from the Enterprise-J belonged to some crew member with an interest in dead species.

History had been changed with the dispersion of the Delphic Expanse (which was now just a regular region of space). Of course the events that happened worked to create the timeline we know from the later (in the timeline) shows. So while the Xindi joined the Federation (and served on the Enterprise J) the Xindi may have not joined the Federation in the primary universe.

History had been changed with the dispersion of the Delphic Expanse (which was now just a regular region of space). Of course the events that happened worked to create the timeline we know from the later (in the timeline) shows. So while the Xindi joined the Federation (and served on the Enterprise J) the Xindi may have not joined the Federation in the primary universe.

But thats just speculation. Any number of explanations could be made.

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No, I think you hit the nail on the head. The Enterprise-J that we saw in "Azati Prime" was from an alternate universe where the Expanse grew unstopped from the 22nd century to engulf most of future Federation space by the 26th. Was the Enterprise-D flying through the Expanse in every episode of TNG? No. That's because the Sphere Builders were stopped by Archer. If he hadn't stopped them, then the future we see in Azati Prime would have come to pass, and not TNG, DS9, or VOY. Obviously the Xindi eventually became Federation members in that alternate timeline, but not in the prime one.

The Xindi might be everywhere, really. Their culture consisted of multiple species, was governed in a somewhat repressive manner, and (largely for the latter reason) managed to remain ignorant of the existence of at least one further, now extinct species. Several other "member species" might have been discovered later on, accounting for the Xindi Starbase of "The Battle" and for the Feline Xindi of "Slaver Weapon" etc.

On the other hand, whatever killed the Avian Xindi might have been built in to the entire spectrum of species originating from their birthworld - or related to the Sphere Builder designs on the culture. Perhaps the species died off one by one soon after the Delphic Expanse was dissipated?

Or perhaps whatever need or will or sentiment connected the different species originally was removed by Archer's antics, and the species still exist, but none of them call themselves Xindi any more. Any of the named but not shown species of TOS, TNG or DS9 could have formerly been part of the Xindi culture, but would now go by a different name.

Obviously the real life reason is that Enterprise was made after those other series.

Other than that though I wondered if anything had ever been put in canon (or even quasi-canon, like a producer or writer or one of the ST bigwigs making a statement in an interview or what have you) by way of explaining why they are never seen "again".

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The Xindi get occasional mention in the 24th century post-series novels. There was a Suliban member of the DTI in Watching the Clock.

As for why they weren't ever seen on-screen, it's a BIG galaxy. We never saw the Remans until Nemesis, although they were supposedly part of the Dominion War. We never see many of the cool background aliens from The Motion Picture, The Voyage Home or JJ's Star Trek in any of the chronologically later series' or movies either.

Back to the novels for a second, Spock's World mentions that the Federation has had contact with 800+ alien races. They can't all be on screen all the time!

The Xindi might be everywhere, really. Their culture consisted of multiple species, was governed in a somewhat repressive manner, and (largely for the latter reason) managed to remain ignorant of the existence of at least one further, now extinct species. Several other "member species" might have been discovered later on, accounting for the Xindi Starbase of "The Battle" and for the Feline Xindi of "Slaver Weapon" etc.

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Kzinti. A totally unrelated species created by Larry Niven for his Known Space universe. Not Xindi. At all.